Update: solvent selection for processing silicone elastomers.

Elastomeric films are used widely throughout industry in applications ranging from coatings for wires to masks on printed circuit boards. In the medical industry alone, membranes and coatings made of silicone elastomer have a variety of specialized applications. Catheters depend on balloons dipped in silicone dispersions, and some wound dressings are based on thin silicone membranes. Prosthetic hands and other body pans gain lifelike detail when they are cast from silicone dispersions that capture minute details of hair follicles and skin wrinkles. In each case, the manufacturing process depends on dispersing the silicone in a solvent so it can be dipped or cast to form a uniformly smooth, flawless surface.

Until recently, silicone dispersions of this type were made by using solvents such as Freon or methyl chloroform (1, 1, 1-trichloroethane) as carriers for the elastomer. However, over the past decade, worldwide concern has developed about the harmful effects of chlorofluorocarbons (CFCs) and similar compounds on the stratospheric ozone layer. In response to that concern, the U.S. and 22 other countries signed a landmark agreement in September of 1987 to limit the production of ozone-depleting compounds. Since the original signing of the "Montreal Protocol on Substances that Deplete the Ozone Layer," growing concern has led to additional agreements that accelerate the phase-out of ozone depleting compounds. The European community plans a total phaseout by the end of 1995, and the U.S. also is striving toward that target.

Many industries have responded by formulating water-based systems that eliminate organic solvents, but in most medical applications, this approach is not viable. Materials that contact body tissue or that transport pharmaceutical fluids or drugs must meet stringent biosafety standards. Typically, converting to water-based systems would require the addition of surfactants and other ingredients that have not been qualified for medical applications or that are not compatible with drugs or body tissues. In addition, because surfactant molecules have both hydrophilic and lipophilic sites, they can be more mobile in the body. For many medical applications, forming dispersions with the aid of a solvent is still the best answer for providing systems with a limited number of ingredients that are compatible with body tissue.

The search for alternatives

Finding appropriate alternatives for organic solvents in medical applications is a complex task. In many cases, it might appear that substituting a familiar solvent would be a simple answer to the problem. For example, isopropyl alcohol is used in manufacturing a number of medical devices. Although isopropyl alcohol is suitable for washing materials, it is not a useful solvent for making a silicone dispersion because under certain circumstances it can react with silicone to depolymerize the elastomer.

Other solvents may serve as substitutes for Freon and methyl chloroform, but they too may be regulated eventually; not in 1994. but still within the 20th century. For short term applications, these solvents may be useful, but there are no materials available that are totally interchangeable with Freon and methyl chloroform. All currently known available substitutions will require changes in processing because of their differences in volatility and flammability compared to Freon and methyl chloroform.

Hydrogenated chlorofluorocarbons (HCFCs) could potentially be used as dispersion agents. but they too are expected to be regulated in the future. These solvents may be acceptable for use as dispersion agents on a short term basis, but it is the responsibility of formulators to test them carefully for use with a particular silicone elastomer and for each individual application.

Alternative solvents for long term

Of the solvents not expected to be affected by regulation in the near future, laboratory tests (ref. 1) of dispersions and cast films suggest four workable alternatives for Freon and methyl chloroform: heptane. hexane, toluene and xylene. Among these, heptane appears to be the most versatile solvent for most applications. For applications of the type described in this article, users should choose the purest form of the solvent available; for example, a USP grade that is certified to be pure and dry. Table 1 summarizes some of the typical properties of silicone dispersions made with each of the four solvents. Heptane has the highest volatility of the four; in actual use, it flashes off, or evaporates, most quickly. For dipping applications. this high volatility is important, because successive layers could be applied to build up thickness without requiring excessive drying time between each coat.

[TABULAR DATA OMITTED]

In film casting applications, it is important that the dispersion has time to level uniformly before the solvent begins to flash off. Heptane produces satisfactory results, as do xylene and toluene, with their lower volatilities. Again, choice of the proper solvent will depend on each individual application. For most applications, laboratory studies suggest that heptane is the best all-around alternative to methyl chloroform: however, for specialized applications, hexane, toluene or xylene may provide optimum results. It is important that users thoroughly review the requirements of their application and process when choosing alternative solvents for silicone dispersions.

Adaptations for processing

Although heptane, hexane, toluene and xylene have physical and chemical properties that make them acceptable for use in silicone dispersions. concern remains over their flammability and over worker exposure issues. Electrical classifications and switching, acceptable solvent vapor concentration per square foot, the use of sprinkling systems and other fire prevention measures should be considered when making a solvent conversion. When altering or requalifying a process to accommodate a different solvent system, users should consult the solvent manufacturer for specific instructions and requirements.

Processing adaptations are highly application dependent. For example, because heptane and the other solvents evaporate more slowly than Freon and methyl chloroform, using them as dispersion carriers may require longer oven times for drying. In this theoretical example, users might choose to increase their capacity by adding shifts or expanding space and adding laminar flow hoods to offset slower production times and maintain production volume.

Changing solvents will require a careful assessment of both the process itself and the finished product. In particular. it is important to determine that all mechanical parts are compatible with the new solvent and that there is no deterioration from contact with the solvent or dispersion. For example, rubber o-rings or gaskets may swell in the presence of heptane, and some plastics may dissolve, become cloudy or otherwise react with particular solvents.

A continuing need for solvents

Silicone dispersions play important roles in the function of a variety of finished products. In some industrial applications, conversion to water-based systems is appropriate. However, for medical applications and other specialized industrial uses, it may be necessary or more efficient to retain a solvent-based system and select an alternative to Freon or methyl chloroform solvent.

In some electrical or electronic applications that require close handwork, it may be necessary to dilute silicone dispersions to decrease their viscosity, while maintaining a system that dries quickly as a result of solvent evaporation. In packaging applications, dispersed silicone fluids may be used to lubricate small components or container closures such as stoppers. In both these cases, it water-based system may not be appropriate because the process requires fast drying as the solvent flashes off.

While conversion to water-based systems is indeed the approach of choice in countless applications, many others are dependent upon the distinct capabilities of solvent-based systems. Adaptin, current processing to accommodate alternative solvents offers industry the flexibility to meet processing requirements and critical environmental concerns.

References

[1.] Internal Dow Coming study.